Anode including graphite and silicon-based material having different diameters and lithium secondary battery including the same

This application claims the benefits of Korean Patent Application No. 10-2018-0127486 filed on Oct. 24, 2018, and Korean Patent Application No. 10-2019-0072305 filed with the Korean Intellectual Property Office on Jun. 18, 2019 with the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference in its entirety.

The present disclosure relates to an anode including graphite and a silicon-based material having different diameters, and a lithium secondary battery including the anode. Specifically, the present disclosure relates to an anode including large-particle graphite, a small-particle silicon-based material, and fine-particle graphite satisfying specific particle size conditions and further carbon nanotube, and a lithium secondary battery including the anode.

The rapid increase in the use of fossil fuels has accelerated the demand for alternative energy sources and clean energy sources, and researches have been actively carried out into power generation and power storage using electrochemistry.

A typical example of an electrochemical device using such electrochemical energy is a secondary battery, which has been increasingly used in various fields.

Recently, technological development and increased demand associated with portable equipment such as portable computers, cellular phones and cameras have brought an increase in the demand for secondary batteries as an energy source. Among these secondary batteries, lithium secondary batteries having high energy density and operating electric potential, long lifespan and low self-discharge have been actively researched and are commercially available and widely used.

In addition, increased interest in environmental issues has led to a great deal of research into electric vehicles, hybrid electric vehicles or the like as alternatives to vehicles using fossil fuels such as gasoline vehicles and diesel vehicles. These electric vehicles and hybrid electric vehicles generally use nickel-metal hydrid secondary batteries as a power source. However, researches using lithium secondary batteries with high energy density and discharge voltage are currently underway and some are commercially available.

Materials including graphite are widely used as an anode active material of lithium secondary batteries. The materials including graphite have an average potential of about 0.2 V (based on Li/Li+) when releasing lithium, and the potential changes relatively uniformly during discharging. This has an advantage that the voltage of the battery is high and constant. Although the graphite materials have an electrical capacity per unit mass as low as 372 mAh/g, the capacity of graphite materials has been improved and now gets close to the theoretical capacity, so it is difficult to further increase the capacity.

For higher capacity of lithium secondary batteries, many anode active materials are being studied. As an anode active material with high capacity, a material which forms an intermetallic compound with lithium, for example, silicon or tin is expected to be a promising anode active material. Particularly, silicon is an alloy type anode active material having a theoretical capacity (4,200 mAh/g) that is at least about 10 times higher than graphite, and is today gaining attention as an anode active material of lithium secondary batteries.

However, silicon-based materials containing silicon causes a large volume change (300% or less) during charging and discharging, resulting in breaking of physical contact between materials and spalling. As a consequence, ionic conductivity, electrical conductivity, and the like drastically decrease, so that practical initial lifetime characteristics tend to reduce sharply.

In order to improve the characteristics of the silicon-based material having a high theoretical capacity, various attempts such as Si/carbon composite have been made in a top-down manner. However, due to a complicated manufacturing process and low yield, they are not sufficient to commercialize it.

Therefore, it is necessary to develop a technique for improving the initial lifetime characteristics while using a silicon-based material as an active material of a lithium secondary battery.

The present invention has been made to solve the above problems and other technical problems that have yet to be resolved.

Specifically, the present disclosure is to provide an anode having improved initial lifetime characteristics while containing a silicon-based material as an active material by including large-particle graphite, a small-particle silicon-based material, and fine-particle graphite satisfying specific particle size conditions and carbon nanotube in the anode material layer, and a lithium secondary battery including the same.

According to an embodiment of the present disclosure, provided is an anode for a lithium secondary battery, wherein an anode material layer is formed on at least one surface of an anode current collector, and

the anode material layer includes large-particle graphite, a small-particle silicon-based material, fine-particle graphite, and carbon nanotube, and satisfies the following conditions 1 to 3:

According to another embodiment of the present disclosure, provided is a lithium secondary battery including the anode for a lithium secondary battery.

The lithium secondary battery including the above-described anode has significantly improved initial lifetime characteristics while containing the silicon-based material as an active material.

Hereinafter, the anode and the lithium secondary battery according to embodiments of the present invention will be described in detail.

The terms are used merely to refer to specific embodiments, and are not intended to restrict the present disclosure unless it is explicitly expressed.

Singular expressions of the present disclosure may include plural expressions unless it is differently expressed contextually.

The terms “include”, “comprise”, and the like of the present disclosure are used to specify certain features, regions, integers, steps, operations, elements, and/or components, and these do not exclude the existence or the addition of other certain features, regions, integers, steps, operations, elements, and/or components.

According to an embodiment of the present disclosure, provided is an anode for a lithium secondary battery, wherein an anode material layer is formed on at least one surface of an anode current collector, and

the anode material layer includes large-particle graphite, a small-particle silicon-based material, fine-particle graphite, and carbon nanotube, and satisfies the following conditions 1 to 3:

The average diameter (D50) is defined as a diameter at 50% of particle size distribution obtained based on a volume of the particles. The average diameter (D50) of the particles may be measured using, for example, a laser diffraction method.

For example, each particle is dispersed in a solution of water/triton X-100, and introduced into a commercially available laser diffraction particle size analyzer (for example, Microtrac S 3500). Thereafter, an ultrasonic wave of about 28 kHz is irradiated for 1 minute at an output of 60 W, and the average diameter (D50) at 50% of the particle size distribution can be calculated from the measuring instrument.

Each of the large-particle graphite and the fine-particle graphite may be at least one selected from the group consisting of natural graphite and artificial graphite.

The natural graphite has excellent adhesion, and the artificial graphite has excellent output characteristics and lifetime characteristics. Therefore, the type and the content ratio thereof may be appropriately selected.

It is not excluded that the above-mentioned large-particle graphite and fine-particle graphite are a mixture of natural graphite and artificial graphite. Thus, the large-particle graphite and the fine-particle graphite may be a mixture of natural graphite and artificial graphite. Alternatively, the large-particle graphite may be artificial graphite and the fine-particle graphite may be natural graphite, or vice versa.

When containing both natural graphite and artificial graphite, a content ratio of natural graphite to artificial graphite may be 5:95 to 95:5, which is preferable in terms of performance of the secondary battery.

The natural graphite may have a specific surface area (BET) of 2 m/g to 8 m/g, or 2.1 m/g to 4 m/g. The artificial graphite may have a specific surface area (BET) of 0.5 m/g to 5 m/g, or 0.6 m/g to 4 m/g.

The specific surface area may be measured by a BET (Brunauer-Emmett-Teller) method. For example, it may be measured by a BET 6-point method according to a nitrogen gas adsorption-flow method a porosimetry analyzer (Belsorp-II mini manufactured by Bell Japan Inc).

The larger specific surface area of the natural graphite, which exhibits excellent adhesion, is preferable. This is because, as the specific surface area is larger, mechanical interlocking effect of inter-particle adhesion through a binder may be sufficiently secured.

The shape of the natural graphite is not limited and may be flake graphite, vein graphite, or amorphous graphite, and specifically vein graphite, or amorphous graphite. More specifically, when a contact area between the particles is increased, a bonding area is increased and thus the adhesion is improved. Therefore, it is preferable that a tap density or a bulk density is large. In addition, it is also preferable that the grain orientation of the natural graphite shows anisotropy, so that the natural graphite may be amorphous graphite.

Meanwhile, the shape of the artificial graphite is not limited and may be a powder type, a flake type, a block type, a plate type, or a rod type. Specifically, in order to exhibit the best output characteristics, a shorter moving distance of lithium ions is better. To shorten the moving distance to the electrode direction, it is preferable that the grain orientation of the artificial graphite shows isotropy, and therefore, the artificial graphite may be in the form of a flake or a plate, more specifically a flake.

The natural graphite may have a tap density of 0.9 g/cc to 1.3 g/cc, more specifically 0.92 g/cc to 1.15 g/cc, and the artificial graphite may have a tap density of 0.7 g/cc to 1.1 g/cc, more specifically 0.8 g/cc to 1.05 g/cc.

The tap density is measured by adding 50 g of a precursor to a 100 cc cylinder for tapping, and then tapping 3000 times using a JV-1000 measuring device (manufactured by COPLEY) and a KYT-4000 measuring device (manufactured by SEISHIN).

When the tap density is too small out of the above range, the contact area between the particles may not be sufficient, so that the adhesion may be deteriorated. When it is too large, tortuosity of the electrode and wettability of the electrolyte may be lowered, so that output characteristics during charging and discharging may be deteriorated, which is not preferable.

Regardless of the kind thereof, the large-particle graphite may have an average diameter D50 (D) of 1 μm to 50 μm, specifically 3 μm to 40 μm, more specifically 5 μm to 30 μm.

When the average diameter (D) of the large-particle graphite is too small, an initial efficiency of the secondary battery may be decreased due to an increase of the specific surface area, so that battery performance may be deteriorated. When the average diameter (D) is too large, rolling property of the electrode may be lowered, the electrode density may become difficult to realize, and the electrode surface layer may become uneven, resulting in low charge-discharge capacity.

The average diameter D50 (D) of the fine-particle graphite may be 0.155Dto 0.414D, or 0.155Dto 0.414Dwith respect to the average diameter D50 (D) of the small-particle silicon-based material to be described below.

The fine-particle graphite needs to satisfy any one of the above two conditions in order to be appropriately located between the large-particle graphite and the small-particle silicon-based material to connect them for improving electron conductivity, in addition to exhibiting the capacity.

When the average particle diameter (D) of the fine-particle graphite is too small, aggregation may occur and it is difficult to uniformly apply the fine-particle graphite onto a current collector when forming an anode material layer. When the average diameter (D) is too large, adhesion may be deteriorated, and the fine-particle graphite cannot effectively penetrate between the large-particle graphite and the silicon-based material. That is, the fine-particle graphite may not sufficiently perform the role of connecting them, and accordingly, electron conductivity may be lowered, which is not effective in improving the initial lifetime characteristics.

More specifically, the average diameter (D) of the fine-particle graphite may be 0.2Dto 0.4Dor 0.2Dto 0.4D.

The small-particle silicon-based material may be at least one selected from the group consisting of Si/C composite, SiO(0<x<2), metal-doped SiO(0<x<2), pure Si, and Si-alloy, and specifically SiO(0<x<2) or metal-doped SiO(0<x<2).

For example, the Si/C composite may have a structure in which a carbon material is coated on a particle surface obtained by firing when carbon is bonded to silicon or silicon oxide particles, a structure in which carbon is dispersed in an atomic state inside silicon particles, or a structure such as the silicon/carbon composite of PCT International Application WO 2005/011030 by the present applicant. The present disclosure is not limited thereto, as long as it is a composite of carbon and silicon material.

The silicon oxide may be 0<x≤1 and includes a structure in which a surface of the silicon oxide is treated with a carbon coating layer or the like.

In addition, the metal-doped SiO(0<x<2) may be doped with at least one metal selected from the group consisting of Li, Mg, Al, Ca and Ti.

When doped as described above, an initial efficiency of the SiOmaterial may be increased by reducing SiOphase, which is irreversible of the SiOmaterial, or by converting it into an electrochemically inactive metal-silicate phase.

The Si-alloy is an alloy of Si with at least one metal selected from the group consisting of Zn, Al, Mn, Ti, Fe, and Sn, and a solid solution, an intermetallic compound, an eutectic alloy therewith may be included. However, the present invention is not limited thereto.

The small-particle silicon-based material may have an average diameter D50 (D) of 0.155Dto 0.414D, specifically 0.2Dto 0.4D.